The present invention relates to a method for fabricating a contact layer on a substrate.
Integrated circuits contain, inter alia, electrical components such as diodes, resistors, transistors, etc., which are connected to one another by electrical lines.
The electrical components are formed for example in a semiconductor substrate made of silicon and have doped diffusion regions as electrically conductive regions. In order to connect the doped diffusion regions to electrical lines, which are usually composed of a metal such as aluminum or copper, an insulating layer preferably made of a boron- and/or phosphorus-doped silicate glass or a silicon oxide is disposed on the substrate surface. Contact holes are formed in the insulating layer and uncover the underlying regions to be connected, such as metalization planes or doped diffusion regions.
Usually, what is called a liner layer made e.g. of titanium, titanium nitride, tantalum or tantalum nitride is deposited, which covers the silicate glass surface, the side walls of the contact hole and the regions to be electrically connected at the bottom of the contact hole, such as the diffusion region. This procedure is described in the Published, European Patent Application EP 0 751 566 A2, for example.
In this case, the liner layer serves as an adhesion layer for the subsequent metalization made of aluminum, tungsten or copper.
In addition, the liner layer has the task of acting as a barrier layer against the diffusion of aluminum, copper or respectively tungsten and gaseous substances thereof in order to protect the diffusion regions.
Furthermore, the liner layer has the task of forming a low-impedance contact between the doped diffusion region and the metal of the contact hole filling.
The deposition of a liner layer is usually carried out by a sputtering process, also called a physical vapor deposition (PVD) process. These processes are specified for example in the document by A. Tolia et al., titled xe2x80x9cIntegrated IMP Ti and MOCVD TiN for 300 mm W Barrier and Liner for Sub 0.18 xcexcm IC Processingxe2x80x9d, SPIE Vol. 3883 (SPIE Conference on Multiload Interconnect Technology III, Santa Clara 1999) pp. 130-135.
Usually, up to approximately 95% of the particles sputtered from the sputtering target have neutral charge, with the result that they cannot be accelerated toward an electrode by an accelerating voltage. Since contact holes often have a large aspect ratio (ratio of the contact hole depth to the contact hole diameter), ionized methods of PVD layer deposition are used. Ionized PVD methods use an alternating magnetic field in which electrons are accelerated and ionize the particles through impacts on their way from the sputtering target to the substrate to be coated. The ionized particles can be accelerated toward the substrate and strike the substrate approximately perpendicularly, with the result that the contact holes are covered at their bottom.
The disadvantage of ionized PVD methods resides in the severe damage to the doped diffusion regions caused by the high striking energy of the particles, which lead to crystal imperfections and crystal dislocations and severe charging effects.
Furthermore, it is disadvantageous that the silicate glass layer can be eroded on a side wall of the contact hole. As a result of which silicon oxide is incorporated from the side wall, at the bottom of the contact hole, on the diffusion region and hence into the contact layer, which leads to an increase in the contact resistance.
It is accordingly an object of the invention to provide a method for fabricating a contact layer that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which a contact layer is deposited having a reduced contact resistance.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating a contact layer. The method includes the steps of providing a substrate in a sputtering chamber, disposing a dielectric layer on the substrate, forming a trench in the dielectric layer, producing a plasma in the sputtering chamber, and sputtering a first contact layer in the trench in a first step. In the sputtering chamber, the number of ionized particles in relation to the total number of particles during the first step is less than 5%. Next, a second contact layer is sputtered onto the first contact layer in a second step, in the sputtering chamber the number of ionized particles in relation to the total number of particles during the second step being greater than 5%.
Ionized particles are understood to be, for example, ionized atoms, ionized crystallites, ionized macromolecules or else ionized clusters, depending on the number and type of bond of the atoms of a particle.
The advantage of the method according to the invention resides in the gentle sputtering of a first contact layer, which serves as protection for the sputtering of a second contact layer. Damage to a doped diffusion region disposed at the bottom of a trench in a substrate can thereby be avoided. In addition, erosion of silicon oxide from the trench side walls is reduced by the protective layer, with the result that less silicon oxide is incorporated into the contact layer at the bottom of the trench. This advantageously reduces the contact resistance, which results in a low-impedance contact. This procedure makes it possible for the ionization rate of the particles to be sputtered to be kept low in the first step. This reduces the damage to the doped diffusion regions at the bottom of the trench and the damage or the silicon oxide erosion on the trench side walls.
In addition, the combination of a first contact layer with a second contact layer which is sputtered by an ionized PVD method improves the coverage on the bottom of the trench, thereby even trenches having a high aspect ratio can be provided with a contact layer.
In an advantageous instance of the method according to the invention, in the second step, an alternating magnetic field is present in the sputtering chamber. This advantageously makes it possible for directed sputtering to be carried out using an electric field, in the course of which sputtering even trenches having a high aspect ratio are covered on the bottom and the side walls with particles to be sputtered. Furthermore, it is advantageous for the pressure in the sputtering chamber to be lower in the first step than in the second step. An advantageous development provides for the pressure in the sputtering chamber to be 2 to 50 times lower in the first step than in the second step. The lower pressure results in that it is possible to reduce the number of ionized particles in the first step in comparison with the second step, which leads to less damage to the doping region in the first step. As a result, it is possible to reduce the ionization rate of the particles to be sputtered, thereby reducing the damage to the doped diffusion regions at the bottom of the trench and the damage or the silicon oxide erosion on the trench side walls. Pressures of 0.1333 Pa to 6.665 Pa (1 to 50 mTorr) are usually used for sputtering processes.
A further advantageous variant of the method according to the invention provides for the substrate to be disposed on a support and an electrode to be disposed on that side of the substrate which faces the support, an alternating voltage being applied to the electrode during the second step. This procedure is referred to as a back sputtering process. In this case, the material sputtered onto the substrate is removed again from the substrate at a low rate. This leads to an improved coverage of the trench bottom, which is an advantage in the case of trenches having a high aspect ratio since the trench bottom is difficult to cover in this case.
It is advantageous, furthermore, that in the sputtering chamber, in the first step, the number of ionized particles in relation to the total number of particles is less than 2%. This procedure reduces damage to the doping region by ionized particles.
It is advantageous, moreover, that in the sputtering chamber, in the second step, the number of ionized particles in relation to the total number of particles is greater than 40%. This procedure makes it possible to achieve an improved coverage of the trench bottom in the second step.
In accordance with another mode of the invention, there is the step of forming the first contact layer and the second contact layer with a substantially conformal layer thickness. In addition, it is preferable that the first and second contact layers are formed from the same chemical element.
In a further advantageous instance of the method according to the invention, the particles to be sputtered have a lower kinetic energy in the first step than in the second step. This procedure likewise reduces damage to the doping region. In a particularly advantageous instance of this method, the particles to be sputtered have a kinetic energy that is less than 10 eV in the first step. In the second step, a kinetic energy of the particles of more than 100 eV is advantageous.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating a contact layer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.